Washington — There are moments, even in highly polarized political times, when the center holds – and counts. This week’s Senate vote on a massive economic recovery plan is one such moment.

Three Republican centrists – the remnant of a once-robust moderate wing of their party – are poised to give Democrats the last few votes they need to pass President Obama’s $800 billion-plus stimulus plan in the Senate.

With a handful of GOP colleagues, they are the likely “swing votes” that could make or break legislation in the Congress for the first years of the Obama administration.

Even with a majority of 58 in the Senate (with one recount pending), Democrats are shy of the 60 votes needed to move major legislation. That’s why Republican moderates like Sens. Susan Collins and Olympia Snowe of Maine and Arlen Specter of Pennsylvania are so crucial to the new president’s agenda.

But they exacted a price. In exchange for their support of the stimulus plan, they negotiated about $110 billion in spending cuts. As in the past, however, GOP mavericks also face the risk of a well-financed party rival in future primary elections.

“There are material risks in the position I’m taking, which may well impact the [2010] primary,” said Senator Specter. “Those thoughts have not escaped my attention.”

Still, he added, “I believe that my duty is to follow my conscience and vote where my faith is in the best interests of the country.”

Role of past GOP moderates

In the Bush years, GOP moderates, also including Sen. George Voinovich (R) of Ohio and former Sen. Lincoln Chafee (R) of Rhode Island, often acted as brakes: In 2003, their votes forced President Bush to trim a proposed $725 billion tax cut to $350 billion. They also consistently blocked legislation to permanently extend those cuts, on the grounds of fiscal discipline.

But for the Obama administration, they’re more likely to be the go-to facilitators on issues ranging from tax and entitlement reform to economic revival and regulation.

For Maine Senators Collins and Snowe, independence and a willingness to work across party lines go with the state’s GOP traditions. Former Republican Sens. Margaret Chase Smith and William Cohen, for whom both Collins and Snowe once worked, were known for their independence. (Senator Cohen also served as President Clinton’s secretary of Defense.) So were former Democratic Sens. Edmund Muskie and George Mitchell. [Editor's note: The original version misspelled Sen. Muskie's first name.]

“There is a long tradition of ignoring both the left wing of the Democratic Party and the right wing of the Republican Party. In Maine, the center holds,” says Christian Potholm, a government professor at Bowdoin College in Brunswick, Maine.“The right wing in Maine plots and plans against them. They did the same for Margaret Chase Smith [and] Bill Cohen, but the far right has no real power in Maine.”

A third-term senator, Snowe won the last general election in 2006 with 74 percent of the vote. She is up for reelection next in 2012. A member of the tax-writing Senate Finance Committee, she has made fiscal discipline her signature issue.

“Olympia Snowe anchors this agreement. She is a rock,” said Sen. Max Baucus (D) of Montana, who chairs the panel.

Collins: moderate to a point

In 2008, Collins bucked a strong anti-Republican tide in the Northeast to win a third term decisively with 60 percent of the vote.

Despite an ad campaign from liberal groups attacking her for her opposition to changing the rules to make it easier to organize unions, or card check, she made card check – an issue supported by President Obama – a centerpiece of her campaign.

It’s a stance that has won Collins respect from some conservative groups that oppose her on issues such as the stimulus. This week, she took the lead, with Sen. Ben Nelson (D) of Nebraska, in negotiating a compromise to enable the Obama stimulus plan to pass the Senate.

“There are issues where people like Collins, Snowe, and Specter don’t vote as Reagan Republicans, such as the massive [stimulus] spending program,” says Grover Norquist, president of Americans for Tax Reform, an antitax advocacy group. “Then, there are issues so damaging to the country and to the freedom movement that they are unforgivable. This is card check.”

Specter takes own road

Fiercely independent, Pennsylvania’s Specter fought back a tough primary challenge from conservative Rep. Pat Toomey in 2004 to win his fifth term. He has voted with Democrats on issues ranging from tax cuts and tort reform to funding for embryonic stem-cell research. [Editor's note: The original version had the wrong date for Sen. Specter's win.]

Aides say his top priorities for working with Mr. Obama include overturning a presidential order limiting federal funding for embryonic stem-cell research and winning confirmation for circuit judges nominated during the Bush years with bipartisan support.

Last week, he and Sen. Tom Harkin (D) of Iowa won a vote on their amendment to the stimulus legislation to add $3 billion in additional funding for the National Institutes of Health, for a total of $6.5 billion. A spokesman says he is still considering his stance on card check.

A handful of other Republican senators worked with Collins on a bipartisan compromise last week, but dropped out of negotiations when Democrats and the Obama team were unwilling to make further concessions.

"At the end of the day, he saw a plan that had too much spending and not enough of it targeted spending," says Ken Lundberg, a spokesman for Sen. Mel Martinez (R) of Florida. But Mr. Martinez has made no secret of the fact that he wants to work with the president on comprehensive immigration reform and the housing situation.

Another negotiator, Sen. George Voinovich (R) of Ohio, dropped out of talks just hours before a final deal was announced on Friday. The "philosophical differences on what the federal government should be doing, especially on school construction, were too far apart," says spokesman Chris Paulitz. But Mr. Voinovich hopes to work with the new president on more infrastructure repair and spending, Mr. Paulitz adds.

– Bridget Huber contributed. This story was updated at 8:57 E.T. on Tuesday, Feb. 10.